text:
Why was the DC-9-80/MD-80 so successful despite being obsolete almost from birth?
The McDonnell Douglas DC-9-80/MD-80 series of narrowbody jetliners was the second-to-last DC-9 major version produced (from 1979 through 1999), the second-largest, the most-produced (just under half of all DC-9s ever produced were of one of the five DC-9-80 minor versions), and the last to be powered by the Pratt & Whitney JT8D low-bypass turbofan.
By 1985, however, the JT8D was already obsolete, with the first small-to-midsize-narrowbody-sized high-bypass turbofans having entered service in the first half of the 1980s, simultaneously offering greater fuel efficiency and lesser noise than the old, outdated JT8D.  Yet, despite this, the DC-9-80 only got more successful as the 1980s wore on and rolled over into the 1990s, with the single best year being 1991, and the years from 1987 through 1992 accounting for over half of all DC-9-80s produced.
Why and how was the DC-9-80 able to be so successful in the late 1980s and onwards, despite its 1960s-era engines rendering it obsolete just a few years after it entered service?

Obsolete? That word, I do not think it means what you think it means.

Why call it a DC-9-80 when most people know it as an MD-80?

Depends on your point of view : When I first traveled in an MD 80, I wondered why it looked so much like a DC-9.

@blacksmith37: That's because it is a DC-9.

-1 for the rollback. While you are right that it is a DC-9, nobody calls it like that. The aircraft is known as MD-80 or Super 80 series, so why remove the MD-80 from the question?

It's simple.  Cheap (development costs amortized decades before) and reliable.  They made money for airlines.  Or, you know, they wouldn't have bought them.  There's more to operating costs than fuel burn, and in any case, fuel prices in the mid 90s were cheaper than at any time since the 20s in constant dollars.  Fuel was so cheap that Air Canada was bringing L1011s out of mothballs in the mid 90s because they could operate them Toronto to LA profitably.

How do you get the mothball smell out of something as big as an L-1011??? ;)

A bigger question is how do you get their little legs apart in the first place?

'because they could operate them profitably' +1. "obsolete" "efficiency" ... those words take a back seat to the Return on Investment. But there might be a slight inconvenience as you will be riding on a cargo plane full of live poultry.

@Mazura - With the mothballs you won't smell the poultry.

@FreeMan just open the windows and fly for a while

First, the airframe and systems were well-engineered and had few flaws and good aerodynamic qualities. Second, the cost to re-engine it with higher bypass ratio engines would have been high, and as such needed to be included in the cost-benefit analysis and balanced against the costs of operating the plane as-is with its original engines. The expected operational lifetime of the airframe would also enter into the analysis in the sense that if a re-engine exercise wouldn't break even before the airframe fatigue lifetime was exceeded, then the plane would be allowed to fly on into oblivion with its original engines and no one would care.
Bear in mind that the opposite is also true- that all the airplanes that used a geared 430 cubic inch 6-cylinder engine (with a 1100 hour TBO) like the twin bonanza, the aero commander 680 variants and various Cessna twins, would get abandoned when the engines were timed-out because the cost of annualing the airframe, rebuilding the engines, the props, and updating the avionics exceeded the value of plane in flying condition.

Even the very oldest DC-9-80s were no more than six years old by 1985 (practically brand-new by airliner standards), with many decades remaining in their airframe lifespans, and McDonnell Douglas did produce a reengined (and slightly stretched) version of the DC-9-80 a few years later.

@Sean economics. If it wasn't economically viable for the airlines to pay for the engine upgrade, then MD would have sunk the development cost for no return. When an airline was buying the MD-90, they were buying a bigger plane with a "full" lifetime, more efficient engines and no need to re-engine in the foreseeable future, therefore a different economic proposition.

@FreeMan I think your missing the point, which is that the question isn't about retrofitting existing planes with new engines at all. It's about why they kept making so many new planes with those old-design engines for so long, and in such numbers, after the better engines became available, instead of switching over much earlier than they did to having all the newly-made ones be made with the new engine designs.

@MatthewNajmon the point still stands. It costs money to reengine an aircraft, whether it's on the flight line or still in the factory. MD would have had to invest money to change the MD-80 to fly with a new engine, and, odds are that they didn't feel it was worth the investment since the MD-90 was rolling out with a newer engine and more seats.

The MD80 was not obsolete at the time of its first flight and the decade thereafter. The market situation for 150 pax aeroplanes around 1980:

Boeing B727-200: first flight 1967. Three JT8D-7/9/11 engines.
MD-80: first flight 1979. Two JT8D-200 engines.

The first generation of JT8D had a bypass ratio of 0.7, the JT8D-200 engines had an increased bypass ratio. From the JT8D wiki page:

Designed to be quieter, cleaner, more efficient, yet more powerful than earlier models, the -200 Series power-plant was re-engineered with a significantly higher bypass ratio (1.74 to 1)

The MD-80 was a market leader and innovator, and was the best proposition 150-seat aeroplane for the airlines at the time. Two engines are more economical than three, higher bypass is more economical than lower bypass. Boeings response in the form of the B737-400 had its first flight only 5 years later, in 1984. The original B737-200 was smaller, made for 100 passengers.

The 737 Classic was officially launched in July 1980 (development having begun the previous year), saw its first orders in March 1981, first flew in February 1984, and entered service in November 1984.  Although Airbus had been doing some conceptual design work on what would eventually become the A320 since 1977, the A320's first orders were in October 1983, it was not launched until a week after the 737 Classic's maiden flight, and the first A320 flew in February 1987, with entry into service only occurring in April 1988.  The A320 most certainly did not spark the 737 Classic's development.

Your question is based on an incorrect assumption.

There's also benefit to parts commonality between aircraft, where if all of the DC-9 variants that an airline was flying were using JT8D engines, then that's one less airframe difference to be concerned about when it comes to maintenance and parts supplies.

Regarding the aspect of obsolence: in contemporary personal land transport, the Otto engine is, by pretty much all means, obsolete. There are very few cases in which the Otto engine outperforms an all-electric drivetrain, but these cases hardly make up more than 10% of the field.
Yet...
The Otto engine prevails. Why? The infrastructure is there. The technology is tried and tested. Yes, you know its crap and that it will mess up the ecosystem of the generations to come, but you don't care, because in your broader perspective of economical and comfort-of-use reasons, it makes sense.
As a reference, you may consider what happened to Boeing when they tried to extend the lifespan of an archaic design while keeping it competetive. I'm guessing that's really the sentence in an answer you were fishing for ;)

And plants love the "fertilizer" CO2, NOx, and SOx, that Ottos spew. But efforts to monitor climate effects since the (colder than normal) 1970s are lauded.

Also define obsolete. The 737, one of the (if not The) most successful airliner in the world was designed about the same time (1960s) and it shows no sign of seeing its production being stopped.
About narrow body = obsolete, I think you will find members of the CRJ (four-across seating [1]) and ERJ families that are not only are narrowbodies but look very similar to a DC9 (five-across seating) down to seat capacity.
About instrumentation on those aircrafts = obsolete, they are upgraded to stay compliant with new FCC regulations. And you will find airliners of the same vintage still flying today.
As others mentioned, there is the operational cost. At a given point in time (so the ratio between them is what we care) it cost 7.2 cents per passenger in operating costs to fly 1 mile on a 100-seat DC9-30 compared to 6.5 cents flying 108-seat Boeing 737-500s. [2] The cost per hour of the DC9-30 is 2053 US dollars[3] which is in the range for the CRJ1000 [1] (1700-2300), a plane launched in 2007 [4].
Finally, would you consider the Boeing 717 [5], i.e. the upgraded DC-9, obsolete?
[1]https://www.aircraftcompare.com/aircraft/bombardier-crj-1000/
[2]https://www.airliners.net/forum/viewtopic.php?t=740289
[3]https://www.airliners.net/forum/viewtopic.php?t=157411
[4]https://en.wikipedia.org/wiki/Bombardier_CRJ700_series#CRJ1000
[5]https://en.wikipedia.org/wiki/Boeing_717

The obsolete part I was talking about is the wasteful low-bypass engines, not the instrumentation or the narrowbody cabin.

The JT8D-9 was replaced with the JT8D-200 when it became the MD-80 and then the Rolls-Royce BR700 after it became the Boeing 717.

Even the JT8D-200, while a considerable improvement upon the earlier versions of the JT8D, is still a loud, fuel-hungry low-bypass engine.  The DC-9 versions after the -80, which did (as you point out) switch to high-bypass engines, are not the focus of this question.

However, even with its (your own words) loud, fuel-hungry low-bypass engine, the cost per hour of the DC9-30 is comparable to a plane launched in 2007 aimed at the same market.

Why wouldn't an airline choose the MD-90, which was more efficient?
Airliners are usually retired because the airframe reaches the end of its useful life, not because the engines are "obsolete." Towards the end of an airliner's production run, the manufacturing process has been optimized and the cost to the manufacturer is lowest. They are willing to give good deals to customers to keep the money rolling in from that product line. It may be desirable for an airline to get a lower upfront cost on an airplane in return for higher running costs on less efficient engines.
As an example, TWA was able to order 24 MD-83s in 1998 and take delivery the next year. Boeing, who had acquired McDonnell Douglas by that time, got more orders to extend production, and TWA (who was in financial trouble at the time) got what was probably a great deal on brand new aircraft on short notice.
Why wouldn't an airline choose another company's options like the Boeing 737-400 or Airbus A320?
Brand and type loyalty is another important factor. Many airlines fly aircraft from a single manufacturer or even type because it reduces the cost of supporting many configurations for training and maintenance.
Not only was the MD-80 series delivered through 1999, but there are still many of them flying to this day, with operators like American Airlines only recently retiring their remaining fleet. They were worth ordering right through the 1990s because they were worth operating well into the 2000s.
The cargo market is much more tolerant of older and less efficient aircraft, but there are still 727s and 737-200s in revenue service, and the 767 is still getting orders despite passenger airlines switching to the 787 years ago.


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How to design the blade twist angle in propeller design?
I am designing a scale propeller for small UAVs and I have computed blade twist distribution for the blade airfoil sections. While I was modelling the propeller I have set the angles to twist about the trailing edge of each section so that this point is collinear. However, the final result looks a bit weird.
I would like to ask whether I have to consider any other point of the airfoil to set for the twist angle (Maybe leading edge or C.G). Below is a screenshot for the final model.

Static and dynamic aeroelastic properties will be important. Therefore a twist location at the quarter chord will (probably) work better.

You should be twisting the profile chord, like in the picture. Why is your tip so small, that is where most thrust is created.
 Source

That is what the optimization tool generated for chord and twist distribution. But i can trim some sections off i suppose.

@george: What tool did you use? I recommend XROTOR - that will give you a proper looking blade.

@PeterKämpf I have used Qmil which jhas similar documentation with XROTOR, so i don't think twist angles and chord lengths are the issues. It might be that i have to consider the twist angle from the center of mass of each airfoil section.

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Why do helicopters not roll over when flying forwards?
Related to this answer: with velocities so different at forward going and rearward going blade, why does the helicopter not roll over? The higher airspeed at the forward going blade should cause more lift, shouldn't it?

Related: Can a helicopter stall?

It seems that the answer to this question can be done in three words: flapping and feathering.  :)

Juan de la Cierva's first autogiro did roll over, twice, and he then applied the principle of blade flapping, a stroke of genius. Flapping is created by allowing the blade to move up and down. Depending on rotor head design this is done in different ways:

By a flapping hinge at the hub, allowing vertical rotation.
By a teetering hinge on two-blade designs, where both blades together are hinged so they can teeter-totter, one flapping up while the other flaps down.

The lift stays equal at both sides because of blade flapping. If the blades can freely travel up- and downwards, the forward going blade indeed experiences more lift, but as an effect of this starts to travel upwards and reduces its angle of attack. The rearward going blade experiences the reverse, less lift makes the blade descend and increases angle of attack. The lift distribution with and without flapping is depicted in this figure from Prouty, Helicopter Performance, Stability and Control:

The local angle of attack distribution of an example helicopter travelling at 115 kts looks like this:

The rearward going blade has a local AoA of 9°, not very far from stall. Increasing airspeed will at one point result in retreating blade stall, with associated loss of lift and increase of drag, and then the helicopter will start to roll over.
Notice that the reverse flow region is not really a big problem in lift creation:

It is close to the disk centre, where lift creation is minimal anyhow.
The loss of lift is reduced because of the increased AoA
Not only lift reverses, drag is negative as well: in this region the airstream actually helps to propel the blade, reducing the required profile power a bit.

Sometimes, it's difficult to understand how the flapping cyclically changes the AoA of the blade. This diagram, taken from the 'Rotary Wing Forum' shows the blade profile as seen from the rotorhead axis and as seen from the real, blade tip axis...

I thought the answer to the asymmetric lift issue is not hinges allowing the blades to rise and fall but hinges on the other axis allowing the blades to move back and forth - essentially varying the rate at which they advance or retreat.

@AnthonyX The lead-lag hinges are there to prevent fatigue failure in the blades: as the blade moves up, the rotational inertia changes, kind of like a figure skater spinning faster when pulling in their arms. The blades do speed up and slow down during the rotation as a consequence of going up and down, but it is a secondary, unwanted effect. Mounting a lead-lag hinge prevents once-per-cycle loads on the blade root from happening.

You are correct that the advancing blade does create more lift than the retreating blade, and you are correct in that flapping counteracts this asymmetric lift. However, this is a much larger effect which comes into play any time there is asymmetric lift across the main rotor:
Gyroscopic Precession
Gyroscopic precession is the (very counter-intuitive) physics phenomenon wherein torque applied to an object with a lot of angular momentum (from the main rotor in this case) actually causes rotation of that object roughly 90° degrees later in that rotation's direction.

Thus, the increased lift on the advancing blade actually results in a pitch-up action, rather than a roll-left, like you might expect.
This even caught Igor Sikorsky off guard during flight testing of Sikorsky's first helicopter:

The design team was not familiar with the fact that a spinning rotor
  had gyroscopic properties (precession) which required an input 90
  degrees in rotation before it became effective.  The VS-300 therefore
  rolled left when the cyclic stick was pushed forward. The initial
  pilots, Igor Sikorsky and Serge Gluhareff, had no idea whether the
  control problems were caused by the helicopter design or pilot
  technique.

copters.com describes the phenomenon in more detail, and Smarter Every Day has a fantastic video explaining it as well.

I don't see the need to invoke a gyroscopic effect. The articulated blades are free to move in flapping, and as lift increases at '3 o'clock' (or 9 o'clock; it depends which way the rotor turns...) the blade flaps upwards, reaching the maximum culmination at 12 o'clock. The retreating blade, lacking lift, behaves precisely in the opposite way, thus sinking from 9 o'clock (or 3 o'clock) to reach the maximum depression at 6 o'clock. All that is caused by aerodynamic forces alone...

Yes what you describe happens when cyclic pitch is applied to the blades. It is the answer to another question though.

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Is it really cost effective to put a motion system underneath a Full Flight Simulator?
Civil full flight simulators are mounted on top of a motion system. It is an expensive system, requires higher and therefore more costly buildings, and requires greater structural integrity of the simulator itself. A cost driver of considerable proportions.
Why do we have to have motion systems on simulators? We get motion information from our central and peripheral vision, right?

Your ears have more say over what your body perceives as "up" than you may think.

I'm not really sure what "cost effective" means here. Full motion simulators are required by regulation for certain types of flight training. If you want the training, you pay for the simulator. You could ask why the regulations exist, as opposed to allowing non-motion simulators, but that's a different question.

Some years ago there was a push from the industry to do away with motion systems due to the cost reasons in the OP.

Here is what a lot of people experiences about simulator: they sit on a chair in front of a desk at home, run some PC flight simulator game with a yoke, and they can do pretty much anything - in the simulator. Many of these simulators are very real, especially with quality addons. Since they fly very good in the simulator, they should be good pilots too - after all, you just read the readings on the instruments then react to them! How hard can that be, huh?
It turns out, we are biologically wired to orient ourselves with our ears. It is the result of evolution, with us staying on the ground for the past million years or so. When we fly, the body sends "wrong" signals to the brain. These signals are interpreted as "instincts", and since we have trusted our body's sensation ever since we're born, they are very hard to fight against. Many new pilots faced trouble reacting to instrument readings, because their biological body is telling them the opposite of what the instrument is indicating, creating doubt.
Full-motion simulators solves this problem. The movement of the simulator itself is, obviously, restricted. However, it is designed with ear-balance in mind. For example, the simulator will slowly right itself to upright over time, but the rate of rotation is just under the detection threshold of the ear-balance mechanism. Therefore, the occupants will not feel the simulator platform resetting itself. This allows pilots to practice with the sensation associated with attitude changes, in a controlled environment, with a much lower risk (and much lower cost) than flying in a real airplane.

Agree, asking if it is cost effective is the wrong question. It is an absolute necessity, and yes it is not cheap.

@Koyovis Asking if it's cost effective is precisely the right question. If there were something different to spend the same amount of money on that would do more to increase aviation safety, it would irresponsible to spend it on this rather than that.

While a simulator with motion simulation is more expensive to own/operate than a simulator without motion simulation it is still cheap compared to a real airliner
So if the regs say a particular type of training can be done on either a real airliner or a full motion simulator then it is likely to be cost-effective to use the full-motion simulator.
And as a bonus you can run scenarios with a high level of realism that would be too dangerous to run on a real airliner. Emergency training in real aircraft has to strike a balance between training pilots to deal with adverse conditions and avoiding accidents during training.

Why do we have to have motion systems on simulators? We get motion information from our central and peripheral vision, right?

A pilot gets motion information from 3 different sources.

His body
His vision
His instruments.

Unfortunately item 2 on that list is sometimes unavailable and item 1 has terrible medium to long term accuracy.
In normal life our brain uses a combination of our bodies motion sensitivity and our perhiperal vision to build up a picture of where we are. In the absense of visual information it will continue to try buliding up a picture of where we are but the accuracy of that picture will quickly degrade.
It's one thing for a pilot to be able to make correct decisions when his only source of motion information is his instruments. It's quite another to make correct decisions when his body and his instruments are apparently in conflict and he has no visual references.

+1 for the first sentence alone! Additionally, crashing in a simulator is far more cost effective than crashing in a real plane, both in monetary cost and in lives lost.

Indeed, which is why you can run scenarios that would be too risky to run or would require compromises to accuracy in the name of safety on the real thing.

The fact that this is how flight simulators are built gives you the answer: yes, it is cost-effective.
It doesn't matter what the cost is, since they are built like that, the cost must be worth it.
It might be more useful to ask: Why are full-motion systems considered necessary to provide adequate simulation for pilot training?
One answer to that would be that only motion systems can provide a reasonable simulation of, for example, turbulence so violent that you can't even focus properly on the instruments.

I do not see how this answers the question. It might fit better as a comment. You first offer an answer in the form of: It it is cost effective, it will be done; it is done, therefore it must be cost effective, which is a logical fallacy. You then go on to offer a legitimate reason why simulators are built and used, but one that does not address whether or not they are cost effective.

This question borders on opinion. Is it cost-effective, as in can a company make/save money by purchasing a full-motion machine to complete required flight training? Certainly it is for some.
But I think what you're really getting at is whether it's cost-effective in terms of reducing accidents. That is really hard to determine. Certainly the authorities think so or they wouldn't require it to fulfill some types of training.
Some might say its usefulness for helping the pilot learn how to use—or more importantly disregard—their vestibular senses is very limited.
The problem is that it is impossible to simulate many of the somatogyral illusions that get pilots in trouble when they lack visual cues. For instance the "leans." This happens in a banked turn. When the plane banks your inner ear feels your body rotate to the side, but in a coordinated turn the the overall feeling of "down" remains in the same direction. There is no way to simulate this. The simulator has to decide whether to tip to simulate the bank—in which case you feel yourself sitting at an angle—or to stay upright simulating the sum of forces. The only solution is to roll the sim to simulate the bank, then slowly roll it back upright hoping to approximate the feeling of gravity+centrifugal force. But that's not what actually happens in flight. Throughout the entire turn "down" never ceases to be perpendicular to the floor.
Lack of solid data can also be a problem. A motion sim is useless at training for unusual attitudes or situations. There is no data to inform the programming for things like high-speed, high-mach stalls or extreme angles of attack because gathering real-world data for these things would be too dangerous. Trying to guess at what it might feel like could be misleading.
On the other hand it can be useful in giving the pilot some practice in dealing with the possibly distracting motions experienced, for example, during a crosswind landing. When pitching or banking a pilot has to deal with the fact that he may not be sitting perfectly straight in his seat or there may be up and down or back and forth motions from turbulence. The pilot has to be sure not to transfer these motions to the controls, so the full-motion simulator can give them some practice at this.
So it really boils down to a subjective judgement call as to whether the expense of a full-motion sim is really justified in terms of accident avoidance.

There is no data to inform the programming for things like high-speed, high-mach stalls or extreme angles of attack because gathering real-world data for these things would be too dangerous. Trying to guess at what it might feel like could be misleading.<<<   As of next year this exact training will be mandatory. Alaska Airlines has got a sim that is certified to recover from fully developed stalls. Data used is windtunnel data plus data from flight recorders of crashed aircraft (unfortunate, but keeping in the diligent tradition of trying to prevent a similar accident).

@Koyovis That is very good to hear. What types of aircraft are they designed for? I think Alaska flies 737s.

They go by category I believe, low wing low tail, low wing T-tail, high wing. Yes the certified sim is for a 737. It is really excellent news and it is tragic that such an important bit of training only gets added to the curriculum after two deadly accidents. Previously only stall prevention and stall onset were trained, not recovery from fully developed stall without ripping the wings off.

@Koyovis IMHO some simple practice hand flying in alternate law would have prevented the AF447 accident from happening.

Yes it may have prevented it. But once the aircraft was in a stall the pilots did not recognise it - had they been trained in it they would have recovered. Prevention is important, as is recuperation if the situation still occurs despite the prevention training.

@Koyovis Very much agreed. In glad they've been able been able to develop a sim that can help with that

@TomMcW Without getting too much into speculation, IIRC one important aspect of what happened on AF447 was that the stall warning went off when the PF commanded nose down, because the AoA went from "so high it can't be valid" to "valid and high". Had the avionics recognized "valid and high, but decreasing" AoA as a valid case, not triggering the stall warning or at least doing so in a distinctly different manner, the PF just might have realized that the right course of action was to keep commanding nose down, allowing recovery. We'll never know for certain what might and might not have helped.

@MichaelKjörling Sent you a chat invite

Is it cost effective to put a motion system underneath a Full Flight Simulator? Yes, because it is a requirement for being able to conduct recurring training, type rating etc. A necessary cost for civil aviation, generating a potential revenue stream.
Why is it a requirement to use a motion system at all? Because of what @kevin says. Without direct stimulation of our inner ear acceleration sensors, the whole "flight" experience is just an interactive wide screen cinema set-up.
Do we need our inner ear to experience motion sensation? Yes and no. Give our brain a moment, and our steady state motion sensation is distilled quite accurately from our peripheral vision. Not from our central vision, where the instruments are.
But any changes in motion are immediately conveyed by our inner ear. That's why we have the sensors there: immediate, direct perception of acceleration. Not of velocity or position, which are integrated signals with the associated 90/180º phase lag, and undefined state of constants.
So our inventory of sensors for position and motion awareness is:

Central, stereoscopic vision for detailed perception of relative position. But stereoscopic vision is not applicable when reading flight instruments.
Peripheral vision for constant velocity detection (central vision is not very good at this, other than from stereoscopic distance perception).
Inner ear acceleration sensors for being alerted to instant changes of velocity.

Motion systems can do a pretty good job at reproducing direct accelerations, until the actuator stroke runs out of course. So for short term accelerations - sustained accelerations require a whole country as stroke. Gravity can be used to a limited extent for simulating sustained acceleration, with some pretty decent effects such as take-off push. An approach flown in a simulator with motion creates much more accurate results, even in the hands of an expert pilot, because the inner ear aids the interpretation from flight instrument data.
For controlling a helicopter in a hover, motion is absolutely essential. Trying to maintain an unstable equilibrium, like standing on top of a large inflatable ball, depends almost exclusively upon inner ear acceleration sensing.
What motion cannot do is reproduce increased load factors such as occurring in co-ordinated turns. Fighter simulators have g-seats and g-suits to do this. And for fighter simulators, it is not considered cost effective to use motion systems, because of the different focus in training tasks. The G-seats recreate the secondary effects from sustained load factor: the pilot is pressed into his seat, so the whole seat is lowered and the seat cushion pumped up. And the G-suit pumps air pressure to the pilots legs, what he experiences in the real aircraft to prevent the blood from being drained from the head.
Motion is pretty awesome, especially the modern electrical systems which are way faster than the old type hydraulic ones. Hydraulics are good for high static forces, but struggle to pump enough oil through the servo valves for fast actuator extensions. Brushless DC electric motors can exert high forces as well, produce instant accelerations, and at 4000 RPM have no problem with high actuator velocities.

Your answer contradicts Kevin's answer: you say the inner ear helps pilots, while this is not generally the case (e.g., loss of spatial awareness in IMC). Another note: a good state estimator (like the brain) does not necessarily have a 90° or 180° phase lag normally associated with a simple integrator; the real problem is that just an accelerometer is not enough to recover the pilot's orientation (or 'undefined state of constants' as you call it).

Yes the inner ear can confuse pilots and get them into a spin or make it difficult to recover from one , however it usually provides a very helpful sense of what is up. An integrator introduces 90º phase lag. Yes an accelerometer cannot provide orientation (position) information, we agree on that. The brain is a good state estimator, and it uses 1st and 2nd derivative signals to get a rapid sense of change.

Complicated question.
A lot have been said about how the human body is receiving these information of motion and they are most of them true and acceptable. But why using Full Flight Simulator with motion. To give the most realistic feedback. Why doing it for small aircraft sometimes less expensive than the simulator. Because the simulator can give you way more possibilities of training, especially on EBT(Evidence Based Training).
But do we really need a FFS for every minutes of Type Rating training. The answer is no. EASA authorize 50% of the time outside a FFS but on a Fixed Based simulator qualified at level FTD 2. (For furthermore explanation go look to the CS-FSTD(A) regulation)
Question is why? Because a lot of the training is just about procedures and doesn't require a full flight to understand how to enter a flight plan or to execute a normal take-off or a normal landing.
So the question is not about do we need motion or not, but what the simulator can let you do in term of training. FFS is mostly used for EBT and other devices are used for procedures learning. That is the real question about the use of a simulator.
I hope it answer the question!
Sources:
EASA CS-FSTD(A): https://www.easa.europa.eu/sites/default/files/dfu/CS-FSTD%28A%29%20Initial%20Issue.pdf
ICAO EBT GUIDE:
https://www.icao.int/SAM/Documents/2014-AQP/EBT%20ICAO%20Manual%20Doc%209995.en.pdf


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What is the difference between the Maximum Glide and Minimum Sink ratios?
In this comment a brief explanation is made of the differences between Max Glide Ratio and Min Sink Ratio:

Sink rate is how much altitude you lose over time, for example, how many feet per second. So minimum sink is the lowest sink rate you can get. Glide ratio is how much distance you can travel per given loss of altitude, for example, how many feet per feet. So max glide ratio is the shallowest angle you can glide at.

However, I don't understand how the lowest sink rate you can get isn't the same as the shallowest angle you can glide at. If I can design wing A to sink at only 1 foot per minute, wouldn't that automatically get me a longer glide ratio from altitude X than wing B that sinks at 2 feet per minute?

No, because one is measured in time - the other is measured in distance. Just because you stay in the air longer doesn't mean you're going further. To me, it looks like the reverse of Vx and Vy. Or the difference between Best Endurance to Best Range.

Rotary wing pilot, so can't comment for fixed wings but on a helicopter, the sink rate and glide ranges are related to rotor efficiency.  Max range is obtained when the rotor is at the bottom of it's power off range, typically 90%, and high airspeed.  This results in a higher rate of descent than normal since you need more power from the rotor, which can only be got by falling quicker. Min rate of descent is typically 100% and 55 kts which is least power required and will keep you in the air longer, but covers a lot less ground horizontally. I imagine it's about efficiency on fixed wing also.

No, as Dan pointed out, the difference is time vs distance. If your theoretical wing that sinks at 2 fpm travels fwd 60 ft in that 1 minute, that would be a better max glide rate than your theoretical 1 fpm sink wing if it only travels 20 ft in that minute (glide ratio of 30:1 vs 20:1 respectively).

Glide ratio is forward speed divided by sink rate.  If I can double forward speed while increasing sink rate by a factor 3/2, that improves glide ratio and hurts sink rate.  You are optimizing for different things, so it is not surprising the optimum conditions are different.

If I can design one wing -- it should be noted that ALL AIRCRAFT have both minimum sink rate and max glide ratio. It's a matter of trim (mainly elevator trim because both are related to the wing's angle of attack). Glider pilots must know the correct trim for min-sink and max glide ratio. He'll use one to optimize flying in thermals and the other to fly great distances to discover new thermals.

@slebetman Yes, all aircraft have both parameters. What he meant was that some aircraft (and their airfoils) are designed to optimize one of those parameters while others are designed to optimize the other due to different planned use cases.

@reirab: I mentioned it because based on his questions I don't think he realize it.

@slebetman yes, I realize that a single wing design will have both a min sink rate and a max glide ratio. I was struggling with understanding why the rate and ratio weren't one in the same/directly linked. I'll reword the question a bit to avoid that confusion

Glide ratio is the ratio of the distance a glider can travel horizontally to the altitude lost in transit. For instance, if a glider can travel 40 miles horizontally while losing one mile of altitude, the glide ratio is 40:1 (typical for a medium-high performance glider). The best glide ratio or maximum glide ratio is simply the best ratio a glider can achieve. Best glide ratio is achieved at a specific airspeed, which varies depending on the glider type. One flies at best glide speed in order to maximize the distance covered. When gliders are flying in a straight line and want to go as far as possible, they fly at best glide speed (typically 55-65kts).
Minimum sink is the minimum vertical speed the glider is capable of flying at in still air (typically 100-200ft/min). Minimum sink is also achieved at a specific airspeed, depending on the glider type (typically 40-50kts). One flies at minimum sink speed to maximize altitude gain when flying in lift. If you're circling in a thermal that is going up at 500ft/min, and your minimum sink speed is 100ft/min, then the glider will go up at 400ft/min.
To review, the key differences are:

Glide ratio specifies how "flat" the glide angle is
Minimum sink specifies the minimum vertical speed the glider can fly at
One flies at best glide speed to maximize the distance covered vs altitude lost
One flies at minimum sink speed to maximize altitude gain in lift
Best glide ratio is achieved at a higher airspeed than minimum sink

Any aircraft can be a glider with its engines out, so this applies to all aircraft.

However, I don't understand how the lowest sink rate you can get isn't the same as the shallowest angle you can glide at.

Remember that minimum sink comes at a lower airspeed than best glide. At minimum sink speed, the glide ratio is worse because even though the aircraft is sinking slower, it is also moving forward slower.

That last paragraph is the best intuitive description I've heard so far.

Wow, this has been sitting without a check mark for a while... After rereading all the answers, I have to agree with @slebetman that your last paragraph finally sank in and made sense. Thanks!

heh, based on my comment, I guess your answer had the minimum sink (in) rate....  /shows himself the door

However, I don't understand how the lowest sink rate you can get isn't the same as the shallowest angle you can glide at.

Look at a Polar Curve:

The lowest sink rate you can get is the maximum value of the curve, as illustrated in this image taken from the wiki article above:

The shallowest angle you can glide at is given by the tangent to the curve that passes through the origin (another image from the wiki article)

If I can design one wing to sink at only 1 foot per minute, wouldn't that automatically get me a longer glide ratio from altitude X, than another wing that sinks at 2 feet per minute?

As we see from the images above, the answer is not a clear yes/no, because it depends on the Glide Polar of the aircraft. Having a wing that sinks at 1 ft/min vs another that sinks at 2 fts/min is no guarantee that the polar is shifted upwards, giving you a better glide ratio. It will depend also on wing loading, as Peter wrote in this answer.

You compare different things. Me can use your example. Wing sinks 1 foot minute and goes forward 10 foot. Another sinks 2 foot minute and goes forward 30 foot. You start with 10 foot. 10x10=100 and 5x30=150. You can see that second wing though sinking at 2 foot per minute was going faster and moved 150ft instead of 100ft. So the angle is shallower.
The same is in real live. If you look at the glider manual it will have two speeds. Max Glide speed and Min Sink(endurance speed, always lower).

Just something to consider (#'s are approximate for a C172SP)
At 60kts, your sink rate is 300fpm:
-  You travel 1.0nm forward over the ground in 1min
-  You descend from 1000’ to 700’ in 1min
-  At this airspeed and sink rate, you have lost 300’ in 1nm of forward travel over the ground, OR, you have lost 300’ in 1min of elapsed time
At 70kts (best glide), your sink rate is 500fpm:
-  You travel 1.2nm in 1min
-  You descend from 1000’ to 500’ in 1 min (i.e, you lose 500’ in 1min of elapsed time, OR you have lost 500’ in 1.2nm of forward travel over the ground)
NOTE:  at 70kts airspeed and a 500fpm sink rate, you would have lost 300’ in .69nm of forward travel over the ground OR you would have lost 300’ in 36 seconds
Airspeed--Sink rate (FPM)--Distance traveled in 1 min (nm)--Distance traveled with the loss of 300’ (nm)--Time elapsed with the loss of 300’ (secs)
60KTS --    300 --  1 --    1 --    60
70KTS --    500 --  1.2 --  .69 --  36
With these notional numbers, with an engine out scenario, I would fly at 60kts (vice 70kts) because I would lose, on average, 300’ after gliding 1nm…if I flew at 70kts, I would lose that same altitude in a shorter distance traveled (.69nm)…I gain more gliding distance per foot of altitude lost at the lower airspeed


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Would this work to reduce turbine engine spool times?
In this answer to a question about reducing engine spool-up time, Peter Kämpf said:

When more thrust/lift is needed, the pilot adjusts pitch and fuel flow to set the desired thrust/lift level. Since the engine is already running at top speed, the change is not delayed by inertial effects.

It seems to me that landing mode or critical mode switch for the engines which would cause the FADEC to spin-up the engine and control power by fuel flow instead would eliminate the lag and increase the likelihood of survival during critical flight regimes.
As an example, encountering a micro-burst during landing will leave the plane wanting power as it flys out of the burst and has a tail-wind, often at low power and low altitude. If, in that situation, the pilot applying additional throttle increased the fuel flow and gave immediate power instead of having to wait for the engines to spool, it seems that would be very advantageous.
Have I understood the concepts correctly, and, if so, what drawbacks to this system might I be missing?

I think Peter was writing about turboprops (and helicopter turbines) - high bypass turbofans don't have variable-pitch fan blades so can't run at full RPM when producing idle thrust. But see Turbomeca Astafan

I got that, @RedGrittyBrick, I was latching on to the fuel flow portion of that statement. I'm not sure, however, if that's possible in a turbine engine.

For the turbine to be running at high speed while the fan was idle, I guess you'd need a variable-ratio gearbox or variable-pitch fanblades - see RR UltraFan.

@FreeMan The point is that in a turboprop, you can set the pitch so that more torque is needed, and RPM is maintained by injecting more fuel (compare pressing the accelerator going uphill in a car). There is no way to do this on current turbofans, so suddenly injecting extra fuel will just blow up your engine.

@sanchises Actually, to be accurate, adding extra fuel in a current turbofan will make it spin faster (which may blow it up, but only if it goes too fast)...

FreeMan, one fallacy in your question is that jet aircraft are at low power settings during landing.  In fact, they are designed to be at relatively high power settings during landing, partially so that they are "spooled up" and the response time to add additional power is not as high.

Short answer: no, none of what you describe is possible with current jet engines. You need more fuel because the angle of attack for your prop is increased, not the other way round.
Let's first go into some (simplified) detail how the turboprop works. A turboprop is basically a gas turbine engine with a propeller stuck on the axle. The propeller is the bit that converts the torque and rpm of the engine shaft into a forward thrust. The amount of forwards force is determined by RPM and blade pitch. This means that, even though you're at high RPM, you can 'feather' your propeller so that it doesn't produce any forward thrust (blades flat). At the same RPM, you can just put your blades so that they produce a lot of thrust. Of course, this comes at the cost of increased drag, so the engine has to 'work harder' to maintain RPM. This, in turn, is achieved by injecting more fuel.
Now, to a turbojet. For a turbojet, the thing that produces thrust is exhaust gas going backwards real fast. This is achieved by burning fuel so that the gas gets hot and expands, so that it wants to get out of there as fast as possible. For this, a jet engine needs a lot of air. It sucks in this air by a series of compressors at the front of the engine. Then, some fuel is injected, and after that there are some turbines that make sure the compressors can, and after that there's a nozzle that converts the leftover energy into backwards gas velocity. Now, we want to increase the thrust. This means that gas has to get out of the back faster. Since the nozzle is generally fixed, this means that you just have to get more air out of the back (just like a garden hose with the tap half open or full open). This means that we'll have to suck in some more air from the front, burn some more fuel to make sure the engine keeps spinning, and more air will come out of the back.
Simply increasing the fuel flow doesn't work. Jet engines operate 'lean' (more air than necessary), and the extra fuel you suddenly inject will burn up with all the excess air, which might blow up your engine. If your engine doesn't blow up, you're left with too little air to burn all the fuel, and you'll get a lot of nasty things like soot, carbon monoxide and unburnt fuel coming out of your exhaust. This means you just inject some extra fuel, wait for the engines to 'spool up' so they suck in more air, and like that gradually increase your engine RPM and linked to that the gas velocity.
Two things remain: why use fixed nozzles? This is because physics dictate that, for engines to be efficient, the exhaust velocity needs to be as high as possible; in practice, the exhaust velocity will be as close as possible to the speed of sound in the hot flue gas. Using a variable nozzle would mean that your engine never works efficiently at half power. Furthermore, it adds complexity, weight and thus costs more.
Another thing: I discussed turbojets, not turbofans. In theory, one could have variable blade pitch on a jet engine. However, remember that the fan is not just for thrust (even though high-bypass engines are generally more efficient), but also to make sure enough air enters the compressors; 'feathering' them would mean that your engine might not get enough air. However, your engine would need a lot of air, because it needs to operate very lean to make sure it can handle the extra fuel flow for when you increase the blade pitch. Furthermore, a variable pitch on all blades would mean a lot of complex engineering, which would make the engine a lot heavier.
Long story short: one could design jet engines with variable pitch, but the above wall of text is only the beginning of why that is an engineering nightmare. Without variable blade pitch or nozzles.

very good answer! Would you think (given a variable nozzle) that an afterburner would do the job?

@rul30 Yes, it would give an instant power boost (in jet fighters, they're used for TOGA purposes). There are some... how shall we call it, practical problems with fitting flame-spitting engines on a passenger aircraft. Let's say the passengers in the back won't like being deaf and slightly crispy due to a 10 feet long fire straight out of hell.


text:
How does the aspect ratio of the vertical stabilizer affect stall speed?

The higher aspect ratio of a vertical tail surface (higher at least when compared to the fuselage) will allow to make the tail smaller, but causes an earlier stall.
  -Peter Kämpf said so here

How does the vertical tail, and more specifically, the aspect ratio of the vertical tail, impact stall?

I think he means the stall of the vertical tail. In general, higher the aspect ratio, earlier the stall.

@Airsick, clicking on Peter's name, would have taken you right to his answer.  Guess I was busy being clever instead of obvious... I've updated to make it a bit less clever and more obvious.

Sorry for the sloppy wording. I hope the ambiguity in my answer is now removed.

Generating lift creates downwash. Downwash reduces the effective angle of attack:

Source
A high aspect ratio reduces the downwash and thus also its reducing effect on the effective angle of attack.
Therefore a high aspect ratio wing will experience a higher $\alpha_{eff}$ than a low aspect ratio wing at the same angle, and thus stall earlier.
The same happens with the rudder, but then with $\beta$ instead of $\alpha$.
EDIT: clarifying alpha - beta similarity
So, in the case of sideslip, we have the following situation:

Source
See the similarity between the $\alpha$-image above, and the $\beta$-image.
The mass flow changes direction, and leaves the end of the rudder (more or less) parallel to the rudder chord line. This deviation in airflow direction causes the force to the left.
So for a normal wing ($\alpha$-image), flow enters straight, leaves going down (downwash) and as a consequence the wings give a force up.
For the rudder ($\beta$-image), the flow enters straight, leaves going to the right (downwash, but then sideways) and as a consequence the rudder give a force to the left.
About the stalling, stalling is not limited to the wings, also rudders can stall if the maximum effective angle they experience is above stalling angle ,and the force generated suddenly drops.
If we compare two rudders both with the same airfoil and the same $\beta_{eff_{stall}}$, say 10°.
The $\beta_{stall}$ will be given by:
$$ \beta_{stall} - \beta_{downwash} = 10° $$
If the $\beta_{downwash}$ is 4° for the low AR wing, and 2° for the high AR wing, it will mean the low AR wing will stall at a $\beta$ of 14° and the wing with the high AR will have a $\beta_{stall}$ of 12°.

I follow the general idea you're getting at, but I'm still confused about A) which way does the down wash from the vertical stabilizer go, and B) I don't understand how this causes the plane to stall.

I've added some more information


text:
How would a tricycle-geared aircraft be recovered from a tip-up?
If improper loading leads to a tip up on a tricycle-geared aircraft:

Image source
what methods are used to right level the aircraft?
I can imagine inflating airbags under the tail, or lifting it with a crane, but what prevents it from slamming down on the nose gear once it passes the tipping point?

You might find your answer on this link, at this site: http://aviation.stackexchange.com/questions/15042/can-passenger-distribution-cause-a-parked-airliner-to-tip

@eduardoguilherme - that Q&A discusses causes for tipping, I'm asking specifically about what is done to right the plane once it's been tipped. Definitely related, though.

RAF squadrons always try to zap a visiting aircraft. To do this, we used a stencil and a can of spray paint. I took it upon myself to zap a Hawker Hunter on the tail, the prized spot.  As I crawled along the spine, the thing tipped up onto the tail. We got it down with about 15 of us pulling on the nose leg.

Here's a YouTube video of an Avro Vulcan being lowered after it tipped. You can't really see how they are doing it. Looks like it tipped due to snow so maybe they just removed the snow. At one point a backhoe appears but it's not clear what role it played.

@TomMcW That's not an operational Vulcan.  It's XL319 at the UKs North East aircraft museum.  A lot of heavy items, including the engines, were removed on scrapping so the weight and balance is all out of whack.  It didn't take much snow to tip her up.  It's hard to see, but they stacked rail track sleepers under the nose wheel (you can see the pile to the left), waited for the snow to melt, then used hydraulic jacks at the rear to tip it up.  The crane was used to ensure that it didn't tip rapidly forward again which could have snapped the fuselage.

While I do not have a specific source to cite, which is always important to reference in regard to any aircraft work, I'm basing this answer on my training as an FAA certified aircraft mechanic.  (There are many more experienced mechanics than myself out there, but alas, none who have done this particular process have arrived yet.)
In general, if you have to work on something that does not already have specified methods from the aircraft manufacturer, the go-to reference is:

AC43.13 1b/2b - Acceptable Methods, Techniques, & Practices of Aircraft Inspection and Repair

But I don't know if that volume covers this mishap.
I can tell you with some certainty that several inspections will be called for on that aircraft.  The skin and structure of the empennage, of course, but also the main gear will almost certainly require a "hard landing check", since the airframe was adversely loaded so much that the nose lifted.
There's every chance that the event will call for a "tail strike" check, which can be extensive.
If the aircraft were tipped forward in an uncontrolled fashion, the nose gear may come down quite hard, but the fact that the forces involved would be unknown (compared, say, to a landing touchdown at a known sink rate and aircraft weight,) would also necessitate a hard-landing inspection of the nose gear.
In a hard-landing event, the fuselage structure may also be compromised, and relevant areas are also checked, as prescribed by the aircraft's particular inspection and maintenance documents.
When an airframe ever does something in an uncontrolled fashion, many inspections will be needed to conservatively account for every possible imagined consequence, and that can cost a great deal of time and money.
All this is preamble to the kind of approach a good mechanic will take to a problem that offers little previous experience.  Every A&P can change a tire, balance a control surface, and check for corrosion, but getting an aircraft back on all wheels safely is not often addressed.
We know a few things:
• The nose gear can support the unloaded nose of the plane (and we know from records what weight range that is).
• Dramatic movements are frowned upon.
• The aircraft was fine before that last load was introduced.
• Personnel safety is a primary concern. (No dangerous heroics!)
Things we don't know:
• How hard you can pull down on the nose gear before damaging something (they were not meant for such loads!)
• Whether the main gear are at or over their load limit already.  (If people were watching the aircraft's weight and balance figures, we wouldn't be in this mess.)
• Whether the fuselage/empennage has been structurally compromised.
• How fast/hard the nose might come down with the weight distribution corrected - too fast?
From these things, we can determine what NOT to do:
• Pile a bunch of baggage handlers in the nose until it tips back down.  Yay, concussions!
• Adding weight (ballast) in general. (Will the airframe be damaged further?)
• Haul downward on the nose with a winch. (Don't tear the nose gear off!)
• Jack up the tail from the rear. (We don't know if it will hold.  Probably, but we don't take chances.)
Variables that could be leveraged:
• Can the cargo be winched back towards the front without endangering anyone?
• Does this model ("type") have fore and aft fuel tanks in the main fuselage, and if so, can fuel be moved between them (or pumped out) for weight shift, in safety?
We're left with two primary goals:
• Decrease the weight moment behind the mains in a controlled fashion, hopefully without adding any weight to the airframe.
• Do not let the nose fall uncontrolled.
My approach would resemble the following, depending on equipment available (or that could be found in the local region):
Provide safe support under the nose of the aircraft that can support the normal full weight of the nose of the aircraft.  We want to ensure the nose cannot come down without our say-so.  A sling is ideal, and the airframe manufacturer will have specifications on how to lift and support the airframe.
In a comment, @TomMcW provided a nice linked visual in this YouTube video:

As seen in the video, the best option would be an overhead crane, with a sling approved for lifting that airframe wrapped under the forward fuselage.
Here's a more extreme example of using a sling to lift an aircraft:

Once the aircraft is in a stable support with minimized risk of unexpected movement, the next phase is to UNweight the rear, in the safest way we can.  That might be to pump out fuel from an aft tank, or winch a cargo container slowly forward, or (the least favorite) manually unload items, using as few personnel as possible.
As weight is removed from the rear, the airframe will weigh down the sling.  You can lower the sling in a slow and controlled way, with the main gear "chocked".
Once the nose is down, sorting out the rest of the cargo and fuel will be trivial, as flight crew can enter and operate internal systems.
But one thing I know for sure: that bird ain't flying today!

Wonderful answer!

Wow - great answer indeed! Thanks for coming along, and way to hit one out of the park on your first answer!

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the average cost/timeframe of obtaining a pilot job at a major passenger airline in the US?
Please note that I'm not asking about getting a certificate good enough for flying a wide-body passenger jet (see related question). Rather, I'm asking about getting from zero flying experience to an actual pilot/co-pilot job at a major US airline (AA/Delta/UA/Southwest).
Perhaps some regulatory organization maintains such a statistics? Or even the airlines themselves?
I'm well aware that people can have various career paths (from ex-military pilots to guys who paid for 10000 flights hours out of their pockets), but with 40,000+ pilots employed by major airlines there must be a statistically-significant median experience.

Well, you're not going to get the job without the certificate (and likely some amount of time working for a smaller "regional airline" carrier). Exactly how long depends on the great unknowable "market forces" -- which airlines are expanding, which are folding, and how many pilots are retiring...

Right now there are over 40,000 pilots/co-pilots working between the 4 biggest airlines alone. I'm sure that there is a certain common scenario of how they got there. Perhaps the FAA compiles some sort of a statistics?

As far as I know that's not something the FAA keeps track of (NBAA or ALPA might track this though) - We might be able to scare up a number for you, but just as a general warning I'd expect pretty wide error bars around any number we give you.

I don't know what the average is, but the minimum is $0 and 0 hours - you can get hired as a trainee pilot without experience.

@RedGrittyBrick not in the US. The only $0 cost 0 time pilot training is courtesy of the USAF but I wouldnt characterize that as a free ride.

The distribution of how pilots get to the majors is bimodal.
If you can get a military pilot slot and give 4-10 years to the USAF, you can then get an FAA ATP and start interviewing at whatever majors are hiring and have your ex-mil buddies on the hiring board. Great way to get in at FedEx or UPS. This costs a few grand for your ATP but that's about it.
If you can't get a military slot, getting up to the point you can instruct from nothing can vary wildly in cost. The accelerated flight training programs will do this for around $70,000. Next you instruct until you hit 135 mins and do either that or instructing. At this point you shouldn't be paying for time anymore. When you get on at a regional you'll sit right seat for a few to many years then you'll upgrade. After about 1.5-2 years in the left seat you'll have the bare minimums to apply at a major.

Your answer is still a bit vague. The military path is clear enough, but I would be curious to see more specific figures for the civilian one. Does the $70,000 include everything or are there more payments? What's "a few to many years" on average - 1/5/10 years?

Its hard to be more clear than that, the cost from 0 to Comm/CFI is entirely dependent on how fast you want to do it, and where you choose to do it.  The 70k figure comes from the accelerated programs what include multi training, which will be beneficial when it comes time to move up the ladder, but greatly increases the training cost.  Likewise "few to many years" depends on the job climate, industry status and company needs.  Get hired at the right time and it could be 2 years.  Get hired at a bad time, it could be 10 or more.

To add to the above, I upgraded between 2.5 and 3 years in the right seat, but I was in the last upgrade class for almost 2 years.  So the difference in hire date by only 2 weeks translated to  3 vs 5 years in the right seat.  It is volatile.


text:
Why are airships not more popular?
There is this question but I am hoping for more general answers to why airships are not more commonplace.
The previous question does bring up the cost of production (mainly because of hi-tech fibers used in body construction) and the high capital costs for constructing hangars for these airships.
However, civilization did operate fleets of these before and I'm just not certain why they have not 'come back' in a meaningful way. For instance, here are just a few uses that I can think of off-hand:

For tourism! Instead of a water-bourne cruise tourists can literally sail all over (I'd pay for that!)...
As the previous question implied, they can be used to carry cargo vast distances (although I am unsure if there is any real economic advantage here).

I know we use them in a small way for various things (military, sports, etc) but why are they not more commonplace?
I imagine the answer is mere economics but at the same time I feel there should be a market if for nothing else than tourism.

No one has really touched on the possibility of a cruise ship style airship. We have mentioned brief aerial tours but I'm talking about a semi-luxury week long trip in one of these things over very remote areas such as Alaska or the Siberia or even a desert... I realise there would not be room for all the luxuries of a cruise ship (pool, casino, etc) but I image about the same luxuries as a small yacht...
I imagine there are a lot of issues with long trips but can airships make unimproved 'port calls'? IE can they just set down for a few hours to let the passengers stretch their legs?

Airships are much more susceptible to bad weather. With an airplane you can pretty much count on it taking off on time and arriving within a couple of minutes of the scheduled time. This allows operators to employ their aircraft with a reliability which an airship will never be capable of. This means that the investment made into an airliner will lead to a predictable stream of revenue and (hopefully) profit, whereas the airship might need to wait several days until it can take off again, or headwind  might delay it by more than a day on a long trip. In this time, it will accumulate interest and fixed cost, but no additional revenue.
Scheduled air services were indeed first done with airships, but as soon as dependable aircraft were available, only a few niches like transatlantic travel were operated with airships. As soon as airplane performance covered those niches, airships became obsolete.
Today the most credible use of airships would be transportation of heavy equipment into remote areas. Tourism could be a second use, but the number of airships needed to cover this market is small. Airships do have some benefits:

no need to build roads or runways
carrying of outsize cargo is no problem
if the airship is big enough, payload can be immense
destinations can be anywhere, not only on a coast or a big river (where regular ships cover this niche more than adequately)

The few cases where the airship could be employed gainfully would not cover the expense of developing and building one. Development cost alone would be similar to the development cost of a new airliner!
But if we look back in history, even at a time when airships were available, big transportation tasks were done by aircraft. Papua has impenetrable jungles, and to transport dredging equipment and personnel to the Morobe goldfield from 1932 on, aircraft were used rather than airships. The huge dredges were dismantled for air transport and flown into Bulolo, where they were re-assembled and put to work. With a Zeppelin, less dismantling would have been needed, but just the complication of keeping the airship balanced when the load is released made this option less attractive.
For the history buffs: To transport very long items like the dredge shafts, a single-engined Junkers Ju-52 (Ju-52/1m) was used. It had a detachable rear fuselage with  automatic couplings for the control rods. Spherical collar screws ensured precise positioning of the two fuselage sections.

Are there any companies today (or ever) that operate airships to deliver that cargo to remote areas?

@MatthewPeters: I do not know any. Even the Zeppelins focused on passenger flights, apart from a short period when they carried bombs. Most of the blimps were used for observation or for advertising, and only the biggest of them could carry passengers.

There is a proposal to make use of airships in the Canadian Arctic. This is because it's impossible to get road, rail or sea transport to many arctic destinations, so the only competition is with fixed-wing aircraft, where airships win for their cost per tonne and speed isn't so much of an issue.

@MatthewPeters: In Germany, there was the company CargoLifter which planned to build, well,  "cargo lifters". The first model CL160 was designed for 160 tons of freight. They got insolvent before building the CL160, but to me, it seems due to mismanagement and too optimistic planning. However, they build a 1:8 prototype "Joey" and a 75 tons balloon. What's left is the biggest self-supporting hall on earth, which now houses a vacation resort: tropical island.

@sweber they are currently offering baloon cranes for construction http://www.cargolifter.com/company/

@Anixx: I wrote about the Cargolifter AG, which definitively got insolvent officially at 2003-7-2. In 2005, some of the old members founded CargoLifter KGaA, which is the company behind your link. However: What are they offering? They have baloons lifting up to 1t. Wow... On the other side the page claims something like they can lift and transport anything to the moon if someone is willing to pay a lot and wait long for the development / assembly of that airship. Actally, they have baloons and may be blue prints for something bigger, but no airships. No references since 2005, just hot air...

@sweber still if a baloon can lift 1t, hold it for a long time and put it precisely, it is a good competitor to helicopters.

@sweber: I was in Brand in 1998. Cargolifter started with a vision, but was soon hijacked by new economy hucksters who ran the company like a cult. What you rightfully call "too optimistic" I saw as gross incompetence. They had started to develop a man-carrying aircraft (Joey) without certifying their development team with the LBA. When I was there, they had claimed to have reached design freeze, but could not tell us what the hull material was. My impression was that they wanted to cash in on the stockmarket without a clear plan what to do a year later.

In the time of the zeppelin, long range travel was for the wealthy and going across the ocean was an adventure in and of itself. People didn't care that much about speed because the only other option was a ship that took weeks.
For cargo, the container ship is much more economical simply because it can take so much more in one go.
The Emma Maersk container ship can take up to 11000 twenty foot containers around the Eurasian continent. She roundtrips between Rotterdam and Hong Kong at a max speed of 47 km/h.

As a point of comparison, it would take roughly 1000 airships the size of the Graf Zeppelin II to carry as much cargo as the Emma Maersk.

How can u use container ship to deliver load into desert or mountains?

@Anixx Deliver the cargo by ship to the nearest port, then rail or road to said desert or mountain. Safer and vastly more economical than using airships. If you really need your cargo there in a hurry, build an improvised runway and use STOL/terrain suitable cargo aircraft.

It's also worth mentioning that airships have more problems with high altitudes than other types of aircraft due to the expansion of the lighter than air lift gas (typically helium) in their bladders, and the cost of said lift gas which needs to be vented.

First let's compare some numbers. Hindenburg-class airships were huge, but only took 90 passengers plus 10 tonnes of additional cargo and cruised at maximum speed of 70 knots. You can put that much load in 737 that can cruise some 450 knots.
If you have something large, heavy and expensive so transport time is important, you can always charter an An-124 that can take up to 150 tonnes or even An-225 that already carried 189 tonnes (it could probably take more though it would need many refuelling stops then). They can also cruise 450 knots, though they are likely to need some refuelling stops with cargo near maximum load. Things like drilling/mining machinery can easily make up the cost of transport in couple of days. And An-124 is capable of landing on unpaved runways, so it can be used for transport to less developed areas.
On the other hand if you have a lot of something, big container ship takes hundred thousand tonnes (the already mentioned Emma Mærsk has payload capacity 154 000 t) and cruises 20-25 knots. That's not that much slower than the airship and it can take almost four orders of magnitude more cargo.
The Hindenburg-class airships were filled with cheap but flammable hydrogen. But after the Hindenburg disaster that is no longer acceptable and any new airship would have to be filled with expensive helium (in fact Hindenburg was originally designed to use helium, but at the time it was only available in the USA and there was embargo for export to Germany).
And even with helium safety record of airships was never particularly good. They fly low and slow which means they are much affected by the weather and they are sensitive to pressure and temperature changes and heating by the Sun, which makes them difficult to handle even in good conditions.

Both the LZ 129 Hindenburg and the LZ 130 were designed for helium, but an American embargo forced the Zeppelin company to go back to hydrogen. And I am unaware that a 737 adds full-size beds and even a grand piano to the frills on board. We must be flying on different airlines.

@PeterKämpf: The information I found regarding hydrogen is somewhat conflicting. Some sources say it was the embargo that forced them to hydrogen, other say it was simply the cost and the fact that they didn't have any incident with hydrogen before the Hindenburg disaster. It's not even consistent across the Wikipedia articles. As for frills, that's not much relevant to comparing the payload capacity, which is still similar. The airship provided a lot of luxury, but the trans-atlantic trip also took 2-3 days (depending on wind) instead of 8 hours in a jet.

My thinking is that the only real market for airships would be tourism (or long endurance operations such as military or comm relays).

@MatthewPeters: The only somewhat successful new airship project seems to be Zeppelin NT. They are much smaller than the large pre-war passenger airships, 5 were built so far and there don't seem to be that much interest. The one that made sight-seeing flights in bay area ceased operations after some 5 years. Some are still operating sight-seeing flights somewhere in Germany, but I don't know whether it's really profitable or just advertising of the ships.

The LZ 128 was designed for hydrogen, but never built after the R 101 disaster. Instead, the larger LZ 129 and LZ 130 were built to have the same lift with helium as the smaller LZ 128 with hydrogen gas. Maybe the Americans now feel sorry for their jingoistic embargo and want to shift the blame for an accident that really did not need to happen.

On the subject of "port calls" to unimproved areas:
I don't know what sort of effect modern technologies (computer autopilots, vectored-thrust engines) would have, but ground handling of Hindenburg-class airships in the 1930s was extremely labor-intensive.  Landing the Hindenburg or Graf Zeppelin required a crew of several hundred to guide it to a mooring mast, and even after mooring, the airship was still somewhat "in flight", requiring a skeleton crew to maintain balance and prevent an incident like this.
An airship can't just "set down" anywhere.  At a bare minimum, you need a mooring mast to keep it from blowing away, and a cleared area around that mast large enough for the airship to swing around with the wind.  For something the size of the Hindenburg, that's a circle a half-kilometer across.  The proposals for using airships for heavy-cargo delivery to remote areas presume that a ground crew is able to reach the area in advance to prepare for the landing.

Absolutely right, and if the payload is heavy, things will become more complicated still. Now the airship has to take up ballast as it is unloading the payload. Water or sand must be plentiful at the remote drop point. It is always funny to watch how in the movies people hop off a ballon, and the balloon just stays on the ground. Won't happen in real life.

You bring up some great points. I am not a balloonist at all but am just curious... The challenges here do not seem too difficult to overcome. Perhaps this is another question but could an airship use anchors similar to waterbourne-ships? Obviously, they would operate in a different technical manner, but if you could have two (1 aft, 1 stern) wouldnt that provide enough pressure to keep the ship steady?

@PeterKämpf: Well, with underslung cargo the airship could descend until the cargo touches down, then descend more until the ropes are not tensed and now the ropes can be untied and the airship is not loaded. And it should be possible by inflating the air ballonets and compressing or releasing some lifting gas. Maintaining the ship in position so it does not drag the cargo along the ground would still be pretty complicated.

@MatthewPeters: No, it couldn't. The anchor line can only bear load along the line. So to resist wind/current moving the ship sideways, it has to also apply some downward force to the ship. If a waterbourne surface ship is pulled down a bit, it's buoyancy increases significantly, so even strong force won't sink it. But there is no similar effect for airship, so it would descent readily until a strong gust of wind would slam it to the ground. Note that submerged submarine would have the same problem; anchors only work well for surface vessels.

Airships can hover (match speed against a ground wind).  No reason they couldn't lower a ground crew, collapsible mooring mast, and ground equipment from below the gondola, at least in steady conditions.

@ZeissIkon, lower a ground crew, collapsible mooring mast, and ground equipment, and simultaneously vent off a few tons of excess lift with sufficient precision to keep either the ground crew or the equipment from falling more than a foot or so as the airship rises.  Hovering an airship requires three-axis control, and near the ground, the engines and control surfaces can only cover two of those axes.

Yep, you''re right.  I was thinking you'd do it with a little breeze, using dynamic lift, but you'd still have to vent a bunch of gas to get heavy, even if you did it gradually before the drop instead of suddenly after or something in between, when the load touches the ground (vent to let the load's weight come onto it; when the cable has near zero load, unlatch and fly away until the mast is up).

For cargo, they're a non-starter. They have much lower capacity than ships and presumably aren't any faster. We already have huge amounts of infrastructure for the shipping industry and would need to build it all again for airships.
Maybe they'd work for tourism. You'd imagine that somebody would already be doing it if there were money in it but, of course, somebody has to be first.

there was a company called Airship Ventures that offered this in the Bay Area up until a few years ago: http://www.sfgate.com/business/article/Airship-Ventures-ends-operations-4041288.php They cited the major reason for closing was rising helium prices

I'd say Airships are not popular because they are highly dangerous in rough weather, many losses and casualties resulted from an Airship caught by an Storm. Many variations about Airship shape exist, including recent versions airfoiled as a lifting fuselage, and making use of aerodinamically generated lift for staying airborne. The Airship  concept has periodical remakes, but as of today, with little success.

Firstly, the drag on an airship is very large, considering its large volume.
Secondly, they are slow and most of the volume is taken up by air/gases like helium etc.
Thirdly, they are much slower than other modes of air transport.
However, an interesting project under development is the Aeroscraft, which is an airship.

Currently a major problem is that the amount of helium on Earth is limited. This lead to the sharp rise of helium prices and it is expected to rise further.
As to use other gases, hydrogen is known to be dangerous and explosion-suspectible. It is also difficult to contain because it leaks envelope of any material due to the small size of its molecules.

Has anyone mentioned the large ground crews required in launching and docking an aircraft? When labor was plentiful and cheap, this wasn't a problem.
Remember, the Hindenburg fire caused many casualties, most of them among the ground crew.

Where do you get the Hindenburg casualties from?  Wikipedia only lists one ground crew fatality, as opposed to 13 passengers and 22 air crew.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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How is "wing thickness" defined?
In the answers to this question about wing thickness, there were many references to wing thickness as a percentage (ranging from 6.5% to 13.5%).
What is that a percentage of?

The wing thickness is basically the distance between the upper and lower surface of the wing (or airfoil).
`
Source: NASA.
The wing thickness is usually given as a percentage of the wing (or airfoil) chord(the distance between the leading and trailing edge). This is to make it a non-dimensional quantity for ease of comparison across different scales.

The picture really helps with understanding. Thanks.

From the image in the answer you link to:

thickness in percent of chord

and the chord (length) is the distance between leading edge and trailing edge of the airfoil of the wing.


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How does early rotation increase the likelihood of a tail strike at take off?
This question indicates that an early rotation can cause a tail strike. What are the (pre-)flight dynamics that would cause this to happen vs what happens when rotating at the proper time?

possible duplicate of In a 747 is it normal for the Vr (takeoff speed) to vary greatly depending on fuel load and ambient temperature?

In my, non-pilot, mind, a tail strike would be caused by over-rotation, which could occur at any speed. What happens when rotating at too slow a speed that would increase the likelihood of over-rotation?

When preparing for a flight, pilots will calculate many things, including their rotation speed. This is the speed in their takeoff roll at which they will start to pitch up, and hopefully the plane lifts off the ground. The rotation speed is dependent on many things such as temperature, altitude, and aircraft weight.
My answer here discusses the lift calculations in relation to rotation. That question covered the importance of speed related to weight. More speed is needed to create the lift to take off with a higher weight.
When the airplane rotates, it increases the angle of attack of the wing, which increases the lift coefficient (if the wing does not stall). The pilots will pitch up at a certain rate, and they do not stop until they reach a climbout attitude higher than what is possible when the main gear is still on the ground (though this may depend on company policy). They count on the plane acheiving enough lift during rotation to leave the ground so that they pass that maximum pitch while in the air.
If pilots underestimate their weight, they will calculate a lower speed than is necessary to provide the lift for takeoff. This means that when they rotate, the plane doesn't have enough airspeed yet to create enough lift to take off. So instead of leaving the ground before fully pitching up, the tail strikes the runway.
It may seem like an easy mistake to make, but thousands of pilots do it every day without a problem.

The pilots will pitch up at a certain rate, and they do not stop until they reach a climbout attitude higher than what is possible when the main gear is still on the ground. That's the part I was looking for to make sense of this.

Interesting. So, is it not part of the procedure to keep an eye on the VSI/altimeter/radio altimeter/out-the-window/anything-that-indicates-positive-climb-rate before exceeding the nose-up attitude that would result in tail strike?

@reirab They can do that, but by the time the PF realizes that the plane isn't taking off, it may be too late to arrest the pitch rate, especially in larger aircraft.

If you rotate too early (that is, with too little airspeed), the plane will need to reach a higher than expected attitude before it begins rising from the runway.
This means that the plane will be closer to the runway than expected, starting from the attitude where it would normally have lifted off.
Thus, when it reaches an attitude where the tail is lower than the main gear (which is supposed to happen routinely during climbout), it may not yet be far enough above the runway that there is room for enough of the tail below a horizontal plane through the bottom of the wheels.

Aerodynamics (the laws of physics) dictate that when a wing produces 'lift' it necessarily produces 'drag' too. It's not difficult to visualize that a force is produced when a wing moves through the air. Going a step further, the vertically upward component of this is lift, and the horizontal rearward component is drag.
Though these forces are active the moment there is forward motion through the air, the flight controls of large jet aircraft become aerodynamically effective around 60kts, the minimum acceptable inflight effectiveness of controls is deemed to be achieved at a minimum speed starting around 110 to 120 kts and above. (below that, it's still steering and brakes included!)
If the pilots erroneously use speeds based on a lower weight than the actual weight, the ability of the airplane to get the nose up using normally expected control forces is compromised as the airflow over the wings is not producing the required lift (yet), and elevator control effectiveness is also lower, nevertheless whatever lift is being produced does cause the nose to rise. Now the scenario is that, from a frontal viewpoint, far more wing area is being presented to the air because the airplane continues to roll on it's wheels rather than starting to get airborne. This results in a larger portion of the already low (due to low speed) aerodynamic force remaining in the direction of the drag component which naturally impedes acceleration thereby keeping it below the correct rotation speed for even longer.
The early rotation can be visualized as a wing mushing through the air and churning it up rather than flying through it in a streamlined manner with a relatively smooth airflow all around.
Now the angle of attack of the wing (~ pitch) has has to be increased still further to produce enough lift to counter the twin effects of insufficient lift due to lower speed, as well as higher drag component. The airplane runs the risk of getting airborne in a high angle of attack high drag configuration seeking even higher pitch - so all around there's a tendency for high pitch and the possibility of a tail strike.
In the earliest days of jetliners, many went off the end of the runway, getting caught out by the compounding of these 2 effects and engine thrust limitations. Those accidents signaled the need for clearcut requirements by the regulations, of take-off calculations based on performance requirements.
The performance calculations do have some inbuilt margins, so the chance of a tail strike if they've calculated the speeds for say 202tons rather than 220tons is less than if they've used speeds for 230tons instead of 320tons.
Be clear that there is no subjective element in take-off procedure and technique. To achieve the legally required take off performance the the thrust setting and speeds are calculated from approved published data and this is only valid provided the airplane is operated as described in the book.
Gross error such as wrong weights used can only be guarded against by doings checks and cross-checks at every stage.

authors: عبدالقوي القطبي


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Heard on LiveATC that ATC wants to "cancel" something. What did the ATC mean by that?
Just heard this on LiveATC:

ATC has cleared the pilot for an ILS approach
ATC then said something to the pilot, to which the pilot replied, "Roger, will cancel with you on the ground, call sign."

What did ATC say to the pilot? What are they cancelling? I didn't catch what ATC said because the audio quality was pretty bad.
The only possible "cancellation" I can think of is the flight plan, but I don't see why they would want to cancel the flight plan if the aircraft has already landed at the destination airport. Any thoughts will be appreciated.

Could you add a link to the audio on LiveATC? It'd be easier for someone to tell what ATC actually said than to guess what it might have been, and that way you might get a definitive answer.

To expand some on DeltaLima's answer:
Pilots on an Instrument Flight Rules (IFR) flight plan are afforded more separation between them and other planes/terrain/obstructions than pilots operating under Visual Flight Rules (VFR). This extends to landing at an uncontrolled airport as well; if an aircraft retains its IFR clearance all the way to the ground, ATC will "protect" the airspace around that airport and will not let any other IFR aircraft arrive at or depart from that airport for a certain amount of time. The airport is considered "one-in one-out" (or I suppose more correctly "one-out one-in") for IFR flights. If the pilot doesn't get in touch to cancel the flight plan search-and-rescue procedures will be initiated.
In this case the controller had a Remote Communications Outlet at the satellite airport. This is an antenna and radio transceiver connected to the radar facility via telephone lines or similar, allowing the pilot to contact the controller directly when on the ground. If an RCO is not available the controller will provide a telephone number that reaches a recorded line at the control position and the pilot can cancel that way.
Most likely the controller said "Report cancellation [of IFR] in the air this frequency or on the ground with me at this other frequency." Or perhaps the pilot had already been switched from the main frequency to the RCO, and was told "Report cancellation on the ground with me." The pilot responded that they would cancel on the ground, instead of in the air before arriving. (If the pilot cancels in the air the airport is not "blocked" for other IFR traffic, which is helpful when there are multiple arrivals in sequence.)
As an aside, it is said that at controlled airports the IFR flight plan is closed "automatically." I had a little trouble understanding what this means when I was first learning about it. It isn't an automated function by the computer that flags the flight plan closed instead of open; rather means that, by default, no one will come looking for you or ask you to report cancelling. Instead the tower controller observes you landing safely, and if you don't will initiate rescue operations.

If the airport is non-towered, then ATC may not be able to have radio contact with the aircraft after landing. However, the flight plan still needs to be cancelled or closed after landing. Usually this is done by the pilot calling ATC on the phone, but in this case it seems ATC offered to do it without the call.
Without closing the flight plan within approximately 30 minutes after the scheduled arrival time, the Flight Services will start looking for you. They will try to contact you by phone call and if that goes unanswered, eventually a search and rescue will be started.
On ATC controlled airports the flight plan will automatically be closed after the aircraft landed.

Some airports have an RCO for Clearance Delivery that can also be used to cancel on the ground.  I notice this mainly at part-time towered airports, for use only when tower is closed.

You seem to be confusing "cancelling IFR" with "closing flightplan". The two are completely unrelated.

@J.Hougaard I confused the two in linking to the question indeed. I've updated the answer. Thank you for the heads up!

@expeditedescent, they aren't completely unrelated...  Because if you don't cancel airborne, and land an a non towered airport, you will have to close your flight plan, right?

@MichaelHall Cancelling IFR means that you are now flying VFR. You are still flying on a flightplan, which needs to be closed at some point. On the other hand, closing a flightplan has nothing to do with whether you are IFR or VFR, the sole purpose is to let search and rescue know that you have arrived, so they do not need to go looking for you

@expeditedescent, that is not true.  If you cancel IFR you have also closed your IFR flight plan.  It does NOT revert to a VFR flight plan (as you suggest) that you later need to take action with FSS or ATC to close.  You are simply VFR, and done with anything having to do with a flight plan at that point.  So, cancelling IFR and closing your (IFR) flight plan are related.

@expeditedescent In the US, canceling IFR closes your IFR flight plan, and now you are flying VFR without any flight plan. That is different from other countries that require a flight plan even for VFR.

I am so Americentric that I again forgot to specify U.S.  (but then neither did Expedite state location...). Thanks for clarifying Stephen!


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As you make a symmetrical airfoil thicker, in which direction does the lift force increase?
I am working on a project and just need to understand one concept that I cannot find an answer to anywhere and hoping you could answer for me.
The project I'm working on is with airfoils and the main on NACA 63-021. In short, from what I have read as the foil get taller/thicker it creates more lift and can handle higher angles of attack as well.
My question to you is: if a foil like this which is symmetrical top and bottom foil shape, what direction of lift is increasing as the foil shape gets thicker? is the lift seen as forward motion lift/thurst?
You help on this would be greatly appreciated!

Try this site. I think some of the side links might get you to lift simulations.  http://airfoiltools.com/airfoil/naca4digit?MNaca4DigitForm%5Bcamber%5D=2&MNaca4DigitForm%5Bposition%5D=40&MNaca4DigitForm%5Bthick%5D=12&MNaca4DigitForm%5BnumPoints%5D=81&MNaca4DigitForm%5BcosSpace%5D=0&MNaca4DigitForm%5BcosSpace%5D=1&MNaca4DigitForm%5BcloseTe%5D=0&yt0=Plot

This site has a tool you use to simulate airflow around airfoils  https://www.engineersrule.com/tutorial-performing-flow-simulation-aerofoil/

Interesting to note that these thick airfoils seem to require a Reynolds number of at least 200,000 to work, which is leading me to believe the Bernoulli top lift of classic aviation lore (circulation) may have both a low and high Reynolds range.  Working on this.  But to answer your question, with a symmetrical, you must have an AOA to create lift.  With symmetricals, you get equal performance for both negative and positive AOA, making them popular for aeobatic aircraft. However, these forgiving airfoils will create more drag at higher speeds.

Lift is defined as the vector component of the total aerodynamic force perpendicular to the freestream velocity vector. Wikipedia about lift
By definition lift can not contribute to thrust or drag, so the direction is fixed.
The sense of the lift vector is opposite to the flow deflection caused by the angle of attack, so the lift increases in that sense.
For your project you might be interested in looking at the lift/drag profile, rather than just at lift.


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What regulation says that a manufacturer must support a fleet above a specific size?
Beechcraft bought back Starship to reduce the size of the flying fleet so they could stop supporting them. What regulation defines the fleet size a manufacturer has to support?

Welcome to aviation.SE! Can you tell us where you got the information about the regulations? This fairly detailed article makes it sound like a purely business decision, it doesn't mention regulations at all (unless I missed it).


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When is a procedure turn not required?
If you are not being radar vectored to the FAF or on a “no PT” transition, are there any other circumstances where a procedure turn that is depicted on an approach chart is not required?

First, I don't believe you need to be vectored to the FAF, just the final approach course, in order to forego a published procedure turn.
Also, you could be cleared "straight-in" when ATC gives you the clearance for the approach.  It may seem subtle, but it makes a difference. Here is the definition in the pilot-controller glossary:

"STRAIGHT-IN APPROACH IFR. An instrument approach wherein final approach is begun without first having executed a procedure turn, not necessarily completed with a straight-in landing or made to straight-in landing minimums."

Also, don't take it literally.  If you approach at a 45 degree angle to the final approach course and are cleared "straight-in", you still approach at a 45 degree angle; there's just no procedure turn.
Using the below approach as an example here the difference would be, "Cleared 0M3 RNAV RWY 2" (Must do the PT) vs. "Cleared straight-in 0M3 RNAV RWY 2" (Shall not do the PT).

Another indicator I know of is notation on the MSA ring that declares a range of radials of approach as "No PT" such as this one.  Note that the MSA and radius on both halves is the same, so the only reason it is split is to declare no PT on the southwest approaching side.


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Why do the A320's centre tank pumps stop working 5 minutes after the low level has been reached?
I'm new here and I'm in training course for Airbus A320. In the training material, it's said that the centre tank pumps are controlled to stop automatically 5 mins after the low level fuel has been reached. Why do they need to wait for a certain amount of time rather than stop right away?

I'm guessing some maneuver, like a climbing turn may uncover the pick-up but there actually be fuel in the tank.

Are they electric pumps or ejectors that run on engine motive flow?

On A320, they are electric pumps and they are jet pumps on A321.
Thanks @Ron. Does it means the centre tank is not divided into small sections, like the wing tanks have inner and outer cells, to stop fuel movement when aircraft maneuvers?

To elaborate on Ron's commment, the pumps are timed because the when the level of the center tank is depleted to the minimum level that can be sensed by a low level sensor, there is usually still residual fuel sloshing around that the pump is able to draw.  For both electric pumps and ejectors, you don't want them sucking air for extended periods, so you want to be able to fully deplete the tank without having the pumps run too long un-ported.
Airbus would have done testing to establish the average time to suck the tank completely dry, added a margin of some sort, then added the timer to the low level shutoff signal with the appropriate delay to achieve that end.

Thanks @John :)


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Is it legal to fly this approach in Bahrain if the VORTAC is out of service?
Can I fly the VOR/DME/ILS RWY 33R approach at Bahrain/Isa Airbase (OBBS) if the OBBS VORTAC is out of service?

Link in question didn't work for me; this one does.